Most often when one wishes to separate a composite material into a plurality of discrete groups, classified by size, the composite material is passed through a plurality of stacked screens of increasingly finer mesh so that the group of material of the largest size is separated first and successively smaller sized groups separated thereafter. This system of separation has some practical problems and is not suited for all applications. For example, because the screens are stacked, it is only useful where there is a great deal of vertical space available. Additionally, because the screens with the more delicate finer mesh are positioned underneath the more durable heavier mesh screens, an imperfection or tear in the finer screens, which more readily occurs, is not easily discernable. Further, the process time required for the material to travel over and through the plurality of screens through many steps renders many processes time consuming and inefficient.
These types of systems also prove undesirable for specific applications. For example, in the oil well drilling industry a drilling fluid, sometimes known as drilling mud, is continuously injected into the well at the drilling location. This fluid not only cleans and lubricates the bit but it also serves as a medium to, by hydraulic pressure, bring the undesirable drilling earthen material, such as sand, crushed shale and the like, to the surface. In some, more simple operations, the undesirable material is then separated from the drilling fluid and the cleaned drilling fluid circulated back into the well.
Oftentimes the formations being drilled will have small fissures or cracks therein. In such instances the drilling fluid can and does flow into these cracks to the expense and detriment of the continuous system. To avoid losses of drilling fluid a process has been developed whereby an additional material, termed lost circulation material, is added to the drilling fluid. This material, which can generally be any fiberous material, then fills or clogs the crevices in the earth and prevents any substantial losses of drilling fluid. Typical of the lost circulation material used are such items as walnut shells, cellophane, sawdust chips or the like.
The addition of the lost circulation material compounds the separating problems because it, like the drilling fluid, is preferably cleaned and recirculated. Thus exiting the well is the drilling fluid of small size, the lost circulation material of a large size, and the undesirable material of a size therebetween, with the largest and smallest of the materials to be recirculated. One proposed solution to this separation problem is nothing more than a conventional two step screening process as shown in U.S. Pat. No. 4,116,288. There the exiting mixture of drilling fluid, lost circulation material and undesirable material is first subjected to a coarse screening to separate the lost circulation material from the drilling fluid and undesirable material which drops to a second finer screen therebelow to separate the drilling fluid from the undesirable material. The drilling fluid and lost circulation material are then reunited for recirculation into the well.
Not only is this system susceptible to the height restrictions and obscure fine screen problem previously described but it also is a slow two step process and even at that, inefficient. Quite often the moist, fiberous lost circulation material will be coated with undesirable material which will not go through the first screen and which is therefore circulated back into the well. In short, no prior art separating equipment is efficiently able to cope with problems such as created by the specific application just described.